This invention relates generally to X-ray phantoms for calibrating CT machines and the like, and specifically to a method of manufacturing an X-ray phantom for simulating bones of various mineral content.
X-rays passing through materials, such as those of the human body, are attenuated. Materials such as bone produce greater attenuation than muscle or other connective tissue and this difference in attenuation can be used to produce an image of these internal body structures, for example, on an X-ray sensitive film exposed by the X-rays passing through the body.
Differences in attenuation, as well as distinguishing between soft tissue and bone, may be used to distinguish between similar tissues of different densities. For example, bone of greater density provides greater attenuation than bone of lesser density. Information on bone density is valuable in the detection and treatment of disease involving bone degeneration, such as osteoporosis.
Conventional radiographic images, as described above, produce a "shadow picture" of internal body structures, the exposure or relative "brightness" at each point in the image representing the total attenuation of all tissue along a line between the X-ray source and that point on the film. As a result, the quantitative measurement of bone density is complicated by the presence of attenuating tissue on either side of the bone.
This problem of isolating the attenuation effect of an internal element of the body, such as a bone, from the surrounding tissue has been largely eliminated with the advent of computerized tomographic imaging systems (CT systems). CT systems use multiple X-ray exposures and a computer "reconstruction" process to generate tomographic or slice images of the human body. The slice images show relative X-ray attenuation, independently, for a variety of volume elements ("voxels") along the cross section of the image. The attenuation is represented by a CT number derived from the reconstruction process and used to define the brightness of the slice image at that point. The CT number for voxels within the bones of the body, as selected with reference to the slice image, thus provides an indication of bone density at those voxels unaffected by surrounding tissue. The advent of CT machines has therefore provided a convenient method of measuring bone density in vitro.
In measuring bone density with a CT system, a region of interest "ROI" is defined, by reference to the CT slice image, around a portion of bone to be measured. Frequently the trabecular bone of the spine is selected for this measurement. The average of the CT numbers of a number of voxels associated with that ROI provide a direct measure of bone density. This value of bone density may be compared to similarly derived values for the same patient at different times.
Unfortunately, the CT numbers produced by a given CT machine scanning a particular object, vary over time. A given object will also produce different CT numbers when scanned by different CT machines. These variations result primarily from changes or differences in the characteristics of the X-ray tubes used to produce the X-rays, primarily energy or spectral characteristics, and from variations in the sensitivity of the detectors which receive and measure the attenuated X-rays.
For this reason, it is typical to calibrate the CT machine used to evaluate bone density with a "phantom" material of accurate and known density placed in proximity to the patient. The calibration is performed by "normalizing" the CT numbers of the image with the CT number of voxels within the phantom.
Ideally, the material of the phantom perfectly mirrors the qualities of the tissue being studied. For example, when bone density is being evaluated, the phantom should have attenuation qualities identical to that of bone. Of particular concern is that the spectral characteristics of the attenuation, the amount of attenuation of the X-rays as a function of X-ray energy or frequency, closely match the tissue being studied. This spectral matching is important because the energy of the X-ray beam between CT machines may vary widely.
Studies have indicated that there are primarily two physical mechanisms accounting for X-ray attenuation: "photoelectric absorption" and "Compton scattering". It follows from this fact that any material may be synthesized by the appropriate combination of two basis materials having relatively different photoelectric absorption and Compton scattering, and by adjusting the total density of the mixture thus produced without changing the ratio of this mixture. For bone, the two basis materials of choice are calcium and aluminum compounds.
A known recipe for making a bone mineral phantom requires mixing of calcium and aluminum compounds into a flexible plastic resin. The ratio between the calcium and aluminum controls the spectral characteristics of the material. Fine, air-filled microspheres are added to the mixture to adjust the total density of the composition.
In practice, the manufacture of bone mineral phantoms according to this recipe is difficult. The primary problem results from the high degree of homogeneity required of the phantom. Ideally, the phantom material must exhibit a homogeneity within three CT numbers, each which represents one-tenth of one percent variation in attenuation. Further this homogeneity must be not only throughout the volume of the phantom but also between different phantoms, i.e., between each batch of the phantom material. An individual voxel measured in the CT image may be only a few millimeters square, and thus this high degree of homogeneity must exist on an extremely small scale.
Each of the ingredients used in constructing the phantom material can be precisely measured, and the use of microspheres containing air allows accurate control of the amount of air introduced for controlling density. Nevertheless, the required degree of homogeneity has been nearly impossible to achieve, especially between separate batches of the material. Of course, batch to batch homogeneity can be achieved, by measuring and selecting the material according to its particular attenuation qualities, however, this is expensive and time consuming.